The present invention relates to a cylinder for a two cycle engine, and, more particularly, to an improved construction of a cylinder for a two cycle engine wherein at least one port formed in the cylinder and opened and closed by a piston reciprocally moved in the cylinder is divided into a plurality of port portions by at least a rib formed in the cylinder and extending longitudinally of the cylinder.
Heretofore, in a cylinder for a two cycle engine, the width of the exhaust port has been increased in order to enhance the performance of the engine and at least a rib is provided in the exhaust port extending longitudinally of the cylinder for preventing the piston ring from catching the upper edge of the exhaust port, thus dividing the port into a plurality (usually two) of port portions each having a narrow width. However, if such a rib is provided, a portion of the rib is made in a swollen form projecting into the interior of the cylinder even though having a honing procedure carried out on the inner surface of the cylinder, thereby tending to cause seizure of the piston. In other words, in the honing process, a plurality of elongated grindstones arranged in parallel to the center line of the cylinder are pressed against the inner surface of the cylinder under a constant load while they are rotated around the center line in contact with the inner surface of the cylinder and, at the same time, they are moved up and down therein. Therefore the area of the inner surface of the cylinder in contact with these grindstones becomes small due to the provision of the port when the grindstones move across the port so that the pressure per unit area is increased, whereas, when the grindstones move across the rib, the area of the inner surface of the cylinder (including the rib) in contact with the grindstones is increased due to the provision of the rib so that the pressure per unit area is reduced thereby resulting in less grinding effect. Thus, a portion of the rib tends to swell and project into the interior of the cylinder so that the accuracy of the cylindrical shape of the inner surface of the cylinder is reduced, tending to cause seizure of the piston in the inner surface of the cylinder. To avoid the above defect, the honing apparatus has been so improved that the inner diameter of the cylinder can be checked in order to vary the pressing force of the grindstones to prevent local swelling of the rib and to achieve a highly accurate cylindrical shape of the inner surface of the cylinder. However, such a honing apparatus is complicated and requires a high cost. Therefore, at present, it is necessary to finish the inner surface of the cylinder by hand after the honing operation, thereby requiring a great time expenditure and high skill resulting in problems of a high cost and variation in the quality.